The present invention relates to the liquefaction of a vapor and, in particular, to the liquefaction of carbon dioxide vapor by the refrigeration contained in liquefied natural gas (LNG).
In numerous instances, it is both physically and economically preferable to transport and distribute materials in liquefied form. This is particularly true, for example, of materials such as carbon dioxide which finds widespread use as a refrigerant in food freezing or chilling processes as well as in the carbonation of beverages. The transportation and distribution of carbon dioxide in liquid form facilitates the handling and storage of this material as liquid carbon dioxide may be readily pumped by means of standard, conventional equipment. In addition, by providing materials such as carbon dioxide in liquid form, the refrigeration content in terms of B.T.U.'s per lb. of material transported renders carbon dioxide economically suitable as a refrigerant. In general, the materials such as carbon dioxide are liquefied by means of conventional cryogenic cycles. It has been found, however, that the equipment required to implement such cycles necessitates substantial capital investments as well as demanding significant amounts of electrical power.
Independent of the of the foregoing is the fact that in recent years the demand for energy producing natural resources, such as natural gas, has increased sharply. The importation of liquefied natural gas (LNG) from foreign sources is being increasingly relied upon in order to meet this demand. One technique for transporting natural gas is to liquefy the same at the point of production and ship the liquefied product to a particular destination, at which point LNG is vaporized and subsequently supplied to most consumers. In addition, it is common to liquefy pipeline gas during periods of slack or lower demands for electrical energy, i.e. the summer months. In this manner, LNG reserves are developed for periods of greater demand at which time LNG is vaporized so as to be useable as a fuel by electrical generating facilities. In each of the foregoing instances the vaporization of LNG, which is commonly maintained at temperatures of -260.degree.F, provides substantial refrigeration capacity that is all too frequently wasted.
In certain instances, attempts have been made to recover the refrigeration available in liquefied gases. One such attempt is generally described in U.S. Pat. No. 3,154,928 wherein a liquefied gas is used to cool brine in a heat exchanger, thereby assisting in the vaporization of the liquefied gas. Such attempts to recover refrigeration in liquefied gas as, for example, described in the aforementioned reference have not proven particularly effective under conditions wherein a careful temperature control of the cooled or refrigerated material is desirable.